Staying in the Senate has helped Hillary politically. She will no longer be a Junior Senator with little experience. That will help her! Don't underestimate Hillary - that woman has trained for years to be in politics and has a good coach (her husband)!
On the flipside...don't underestimate Middle America.
Hillary promised to serve her full 6 year term when she ran for the Senate. Her 1st term isn't "Staying in the Senate" it is fulfilling a commitment she can't run out on. "Staying in the Senate" is running for a second term. I for 1 think it is not necessarily a given she will run again. I'm waiting for her to say "I'm running".